CFD investigation on the maximum lift coefficient degradation of rough airfoils

Ice accretion can reduce the performance of aircraft's wings, which results in higher fuel consumption and risk of accidents. Experiments proved that even in its very earlier stages (increased roughness), icing could cause a reduction of 25% in the maximum lift, and an increase of 90% in drag of an aspect ratio 6 wing. In this work, we propose a correlation to predict the degradation of the maximum lift coefficient caused by roughness effects on flows over two airfoils, a NACA 0012 and a model 5–6. In addition, a second correlation is proposed to find the minimum Reynolds number that are useful for higher Reynolds number applications when roughness is considered. The SA roughness extension is implemented into an open-source code called SU2. The verification and validation of the implementation is performed in two steps. First, the behavior of the flow over a smooth NACA 0012 is investigated to confirm whether the implementation has no influence on the original model when roughness is not activated. Then, roughness is activated, and estimations of lift coefficients and velocity profiles inside the boundary layer are evaluated and compared to numerical and experimental results. Finally, investigations on the maximum lift coefficient reduction caused by different equivalent sand grain roughness heights and Reynolds numbers are performed. Our results demonstrated that, for the equivalent sand grain roughness heights investigated, the variation of sufficiently small heights has no significant influence on the maximum lift coefficient degradation. Moreover, when roughness is continuously increased, a saturation point seems to be approached, in which the variation of the maximum lift coefficient degradation is reduced. We noticed that although the reduction of the maximum lift coefficient caused by different equivalent sand-grain roughness heights and Reynolds number present similar behavior, they fall into different curve formats.